This invention relates generally to a method for transmitting data through a communication network at radio frequencies in the presence of interference, and more particularly to a method for synchronizing information in a packet communication format on a plurality of frequencies without use of a central timing source of synchronization.
Packet communications is a form of data communications whereby segments or packets of data are self contained as to target address, source address and information content. Conventionally, packets are routed with error checking and confirmation of receipt directly or relayed via relay stations, or intermediate nodes, between a source node or station and a destination node or station on a frequency which is known at all times to nodes within direct communication. Communication on certain frequencies may be restricted in duration in accordance with frequency allocation and bandwidth utilization rules and requirements. Such restrictions may be imposed by a licensing authority, such as the U.S. Federal Communications Commission (FCC). For example, in the 902-928 MHz frequency band, the FCC has proposed a rule that continuous transmission by a single transmitter of no more than 1 watt rf output power on any one channel be of no more than 400 ms duration each 30 seconds, and that at least some if not all other channels be selected prior to retransmission on the same frequency (FCC Rules, Part 15.247). Communication between any given pair of transceivers on a single frequency is thus restricted to packets of information which can be communicated in less than 400 ms, and means must be provided to accommodate communication on other frequencies.
In order to accommodate the transmission and reception of significant amounts of information in a reasonable period of time, frequency-agile transceivers have been proposed in which each transceiver changes frequency according to a pseudo-random pattern among a plurality of channels. Such systems are called frequency-hopping spread spectrum systems and are not to be confused with direct sequence spread spectrum systems wherein a pseudo-random code is used to modulate an information-bearing carrier over a broad band.
A packet-based frequency-agile system has special difficulty in synchronizing signals, as each transceiver typically would have no knowledge of the other transceivers within communication range. What is therefore needed is a technique for acquisition and maintenance of synchronization of information transmission and reception in a frequency-agile packet communication network.
The following patents were uncovered in a search of the records of the U.S. Patent and Trademark Records:
Smith, U.S. Pat. No. 4,850,036, issued Jul. 18, 1989, for RADIO COMMUNICATION SYSTEM USING SYNCHRONOUS FREQUENCY HOPPING discloses a frequency-hopping radio system in which a master station or control unit transmits a startup message to a plurality of slave stations on a preselected frequency. The startup message indicates to each slave station a frequency-hopping sequence. Therein, all transmissions must be synchronized to the control unit at all times to preclude interference among slaves. Such a system is impractical in a peer to peer network where there is no global master station or global timing.
Ham et al., U.S. Pat. No. 4,837,823, issued Jun. 6, 1989, for COMMUNICATION SYSTEM describes a frequency-hopping communication system wherein synchronization information is transmitted over a spread of frequencies of a type which differs from the type used for data communication, and a counter at each receiver is used to keep track of sequence and synchronization. Such a scheme as therein described requires substantial acquisition time and overhead.
Pyatt et al., U.S. Pat. No. 4,807,248, issued Feb. 21, 1989, for AUTOMATIC RESYNCHRONIZATION TECHNIQUE describes a method for resynchronizing frequency hopping communication systems. Resynchronization frames are transmitted interspersed with data frames following initial synchronization. This technique, which can be adapted to packet communication between two stations, requires additional signals to be sent as well as a master-slave relationship between stations.
O'Connor et al., U.S. Pat. No. 4,677,617, issued Jun. 30, 1987, for RAPID FREQUENCY-HOPPING TIME SYNCHRONIZATION describes a frequency-hopping radio system in which a master station or control unit transmits a unique synchronization code for each time interval relative to a reference startup time. This system also proposes a master-slave relationship among stations.
Hardt et al., U.S. Pat. No. 4,601,043, issued Jul. 15, 1986, for DIGITAL COMMUNICATIONS SOFTWARE CONTROL SYSTEM describes a frequency hopping system with a USART and a bit/sync tracking circuit.
Mimken, U.S. Pat. No. 4,558,453, issued Dec. 10, 1985, for SYNCHRONIZATION METHOD AND FREQUENCY HOPPING SYSTEM describes a frequency hopping system in which keying of a transmitter automatically initiates a multiple cycle sync acquisition signal and wherein a sync maintenance signal is periodically transmitted.
Other commercial or proposed commercial systems which have come to the attention of the applicant as a result of FCC consideration are as follows:
O'Neill Communications "LAWN" system operates a packet communication system based on AX.25 link layer protocols. The system uses four channels at a data rate of 38.4 kbs in the 900 MHz band in a multipoint to multipoint scheme.
LifePoint System operates a multipoint to point event reporting system using unacknowledged packets. The system occupies 800 kHz centered at 908 MHz. The system is used for alarm systems.
Telesytems RadioNet SST is a system which operates at a frequency of 915 MHz at a variety of transmission rates and power levels. The system operates without a link layer protocol.